A. F. Rogers — Law sonite from California. 105 



Art. V. — Lawsonite from the Central Coast Ranges of 

 California ; by Austin F. Rogers. 



Of the new rock-forming minerals described within the last 

 decade or two, lawsonite, a basic calcium aluminum silicate, is 

 one of the most interesting if not important. Though usually 

 an accessory constituent, in some cases it proves to be an essen- 

 tial constituent of certain rocks. The type locality of lawsonite 

 is Tiburon Peninsula in Marin County, California, where it 

 was found by Ransome 1 and by Palache* 2 almost simultaneously. 

 Since its discovery in 1894 it has proved to be a widely dis- 

 tributed mineral, for it has been observed by Franchi 3 in the 

 glaucophane rocks in the Piedmont Alps, by Viola 4 in the 

 diabases and gabbros of southern Italy, by Lacroix 5 in the 

 glaucophane schists of Corsica and the gabbros of Corsica and 

 New Caledonia, by Manasse 6 in the island of Gorgona, and by 

 Termier 7 in the glaucophane schists of the department of 

 Hautes-Alpes. In the coast ranges of California lawsonite is a 

 fairly common constituent of the metamorphic rocks of the 

 Franciscan series, as has been pointed out by J. P. Smith 5 and 

 by Eakle 9 . 



Lawsonite occurs in orthorhombic crystals of simple habit, 

 which are usually tabular or prismatic. The common forms 

 are m\110\, c\001}, b{0lQ\, and tfjOll}, with mm{110: 110} 

 = 67° 16', and dd (Oil : Oil) = 72° 53J'. Hillebrand and Schal- 

 ler 10 give in addition the forms ^{041}, r\ 221}, and sj331}. 

 The specific gravity is 3' 12 and the hardness 7-8. The color 

 is grayish -white to bluish-gray. 



The optical orientation of lawsonite is a = a, /3=£>, y=e. The 

 indices of refraction as given by Ransome 1 are fly = 1*684, 

 n/?=l*669, wa = l*665 ; ny-n a ='019. Basal sections are usually 

 rhombic in outline, with first order interference colors, while 

 the other sections are rectangular with second order interfer- 

 ence colors. The basal sections often show polysynthetic twin- 

 ning with m\110\ as twin-plane. 



Chemically, lawsonite is interesting because it corresponds to 

 anorthite with two molecules of water added. The empirical 

 formula is H 4 CaAl 2 Si 2 O in , as proved by the following analyses 

 which are the only complete analyses available. The last 

 analysis of this list (No. VI) is a new one of lawsonite from 

 Alameda County, California, which is described beyond. 



There are two characteristic kinds of occurrence of law- 

 sonite. (1) As a secondary mineral in slightly altered or saus- 

 suritized diorites and gabbros. California, Corsica, Piedmont 

 Alps, Gorgona, and New Caledonia. (2) As an accessory or 



